In mobile communications, it is necessary to compress and encode digital information such as speech and images for efficient use of radio channel capacity and storage media for radio waves, and many coding and decoding schemes have been developed so far.
Among these, the performance of speech coding technology has been improved significantly by the fundamental scheme of “CELP (Code Excited Linear Prediction),” which skillfully adopts vector quantization by modeling the vocal tract system of speech. Further, the performance of sound coding technology such as audio coding has been improved significantly by transform coding techniques (such as MPEG-standard ACC and MP3).
On the other hand, a scalable codec, the standardization of which is in progress by ITU-T (International Telecommunication Union—Telecommunication Standardization Sector) and others, is designed to cover from the conventional speech band (300 Hz to 3.4 kHz) to wideband (up to 7 kHz), with its bit rate set as high as up to approximately 32 kbps. That is, a wideband codec has to even apply a certain degree of coding to audio and therefore cannot be supported by only conventional, low-bit-rate speech coding methods based on the human voice model, such as CELP. Now, ITU-T standard G.729.1, declared earlier as a recommendation, uses an audio codec coding scheme of transform coding, to encode speech of wideband and above.
Patent Document 1 discloses a coding scheme utilizing spectral parameters and pitch parameters, whereby an orthogonal transform and coding of a signal acquired by inverse-filtering a speech signal are performed based on spectral parameters, and furthermore discloses, as an example of coding, a coding method based on codebooks of algebraic structures.
Patent Document 2 discloses a coding scheme of dividing a signal into the linear prediction parameters and the residual components, performing quadrature transform of the residual components and normalizing the residual waveform by the power, and then quantizing the gain and the normalized residue. Further, Patent Document 2 discloses vector quantization as a quantization method for normalized residue.
Non-Patent Document 1 discloses a coding method based on an algebraic codebook formed with improved excitation spectrums in TCX (i.e. a fundamental coding scheme modeled with an excitation subjected to transform coding and filtering of spectral parameters), and this coding method is adopted in ITU-T standard G.729.1.
Non-Patent Document 2 discloses description of the MPEG-standard scheme, “TC-WVQ.” This scheme is also used to transform linear prediction residue into a spectrum and perform vector quantization of the spectrum, using the DCT (Discrete Cosine Transform) as the orthogonal transform method.
By means of the above four prior arts, it is possible to apply, to coding, quantization of spectral parameters such as linear prediction parameters, which is part of a useful coding technique of speech signals, thereby enabling the efficiency and low rate of audio coding to be realized.    Patent Document 1: Japanese Patent Application Laid-Open No. HEI10-260698    Patent Document 2: Japanese Patent Application Laid-Open No. HEI07-261800    Non-Patent Document 1: Xie, Adoul, “EMBEDDED ALGEBRAIC VECTOR QUANTIZERS (EAVQ) WITH APPLICATION TO WIDEBAND SPEECH CODING” ICASSP'96    Non-Patent Document 2: Moriya, Honda, “Transform Coding of Speech Using a Weighted Vector Quantizer” IEEE journal on selected areas in communications, Vol. 6, No. 2, February 1988